Moisture-Electric–Moisture-Sensitive Heterostructure Triggered Proton Hopping for Quality-Enhancing Moist-Electric Generator

Highlights An efficient moist-electric generator with ultra-fast electric response to moisture is achieved by triggering Grotthuss protons hopping in the moisture-electric–moisture-sensitive heterostructure. The moist-electric generator produces a quick response (0.435 s), an unprecedented ultra-fast response rate of 972.4 mV s−1 to alternating moisture stimulation and stable output for 8 h. An obstructive sleep apnea hypoventilation syndrome diagnostic system based on a moist-electric generator was developed to monitor hypopnea and apnea in real time and successfully diagnose them with early warning. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-023-01260-w.


Note S2 Characterization and Electrical Output Performance of Graphene Oxide (GO) Film
Obtained by the method of solvent evaporation induced self-assembly, the GO as the electricity-generating layer has good flexibility (Fig. S1).The microstructure and morphology of the surface and cross-section of the GO film were investigated by scanning electron microscopy (SEM), showing the smooth surface and ordered layered stacking structure of GO (Fig. S2).In addition, from the energy dispersive spectroscopy (EDS) spectrum (Fig. S2d), the weight ratio of oxygen element is as high as 38%, which means that the GO film contains abundant oxygencontaining functional groups.
The electrical output performance of the GO moisture-electric (ME) generator (GOMEG) was tested after placing a microporous gold electrode on the top layer of GO directly dried on ITO conductive glass (Fig. S3a).Moisture is carried by N2 flowing through deionized water, and the RH of the test system is changed by adjusting the flow rate of N2, as shown in Fig. S10b.A voltage of 0.4 V and a current of 120 nA were generated in GOMEG under 90% RH, while the electrical output gradually decreased under the stimulation of dry N2 (Fig. S3c, d).According to our previous report, this phenomenon is caused by the moisture gradient brought by asymmetric moisture stimulation on both sides of GO [S1].The stacking structure of layered GO hinders the diffusion process of water molecules, and the side close to the moisture absorbs more water vapor than the side farther away, resulting in the hysteresis of water transfer (Fig. S3b).In addition, the lower electrode, acting as a moisture-isolating substrate, further strengthens the asymmetry of water content inside GO, and the GO on the side of the moisture-isolating substrate cannot effectively contact moisture, while the side far from the moisture-isolating substrate can be sufficiently wetted.The upper layer of the GO film with excellent hydrophilicity is directly exposed to moisture, and oxygen-containing functional groups, such as carboxyl groups, interact with water molecules to ionize carboxylate and H + ions.The interaction of moisture and GO is hindered by the moisture-isolating substrate, so there are very few H + ions in the lower layer.Driven by the concentration difference, H + ions diffuse from the upper layer to the lower layer with water molecules, and the carboxylate groups are fixed on the carbon skeleton of GO and cannot move.H + ions and carboxylate recombine with the withdrawal of moisture, and the output voltage and current drop to the initial state.Therefore, the mobile restricted functional groups and the directionally mobile H + ions realize the self-generated electricity effect of the GO film.Moreover, the output voltage and current of GOMEG vary with external resistance, as shown in Fig. 3Se, f.When the external resistance is greater than 10 5 Ω, the output current decreases and drops to 0 at R=10 8 Ω.The output voltage drops when the external resistance exceeds 10 7 Ω and becomes 0 at R=10 9 Ω.
To further verify the principle that the moisture gradient causes GO to generate electricity, both sides of the GO film without the moisture-isolating substrate were exposed to moisture simultaneously (Fig. S4a).The output voltage of the symmetric moisture-stimulated GO film is almost zero (Fig. S4c) due to the same degree of hydration inside the GO resulting in no ion migration (Fig. S4b).Furthermore, the left-right asymmetric moisture stimulation on GO induced a lateral moisture gradient (Fig. S4d, e) and GO also generated a voltage of 0.4 V (Fig. S4f).

Note S3 Characterization and Electrical Output Performance of ZnO
ZnO is a crystal with a hexagonal wurtzite structure (Fig. S7), and the morphology is irregular nanosheets with a size of 200-500 nm (Fig. S8).Irregular ZnO nanosheets are stacked together to form many holes that facilitate the entry of water molecules.A ZnO layer of the same size as that on the graphene oxide-zinc oxide ME generator (GZMEG) was screen-printed between the two electrodes, and then the ME performance of the ZnO layer was tested (Fig. S9a).Both the voltage Nano-Micro Letters S3/S24 and current outputs of the ZnO coating are 0, which indicates that ZnO has no ME characteristics (Fig. S9b, c).

Note S4 Characterization and Electrical Output Performance of GZMEG
The process of preparing GZMEG by step-by-step screen printing is shown in Fig. S10a.The RH control system that measures the ME output of MEG consists of external N2 sources, a sealed deionized water bottle, a sealed constant RH bottle, a Keithley 2612 multimeter, and a RH detector.The moisture is carried by N2 through deionized water to control the RH of the MEG surface.The Keithley multimeter connected to the electrodes of the MEG measures the electrical output, and the RH detector indicates the real-time RH.
SEM images of the GO-ZnO heterostructure interface show that the ZnO coating with loose pores is in good contact with the GO film (Fig. S5).The water contact angle of ZnO prepared by powder tablet press is even close to 0° (Fig. S11a), representing that ZnO is super-hydrophilic, so the ZnO coating does not hinder the entry of water into the GO film.Moreover, the water contact angle of GO film is 51° (Fig. S11b), indicating that it has good hygroscopicity and can generate electricity when interacting with water.
The response speed of GZMEG to RH changes is greatly improved compared with GOMEG, as shown in Figs.S12d, e.The instantaneous response rate of GZMEG in the humidification process can reach 0.5 V/s, and the response rate of the dehumidification process even reaches 2 V/s, while the total response rate of GOMEG is close to 0 V/s.In addition, the current response of GZMEG to moisture is twice as fast as GOMEG (Fig. S13a, b).It is worth noting that the response of GZEMG to dry N2 has a super-strong improvement, and its instantaneous response speed is as high as 450 nA/s (Fig. S13c).
The X-Ray Diffraction (XRD) pattern, Fourier Transform Infrared (FT-IR) spectrum, and SEM images of GO in the initial state of GZMEG and after 100 cycles of ME testing are shown in Figs.S14-S16.The XRD confirms the representative peak at 2θ = 11.58°,indicating a larger interlayer spacing in GO (ca.0.76 nm) than in pristine graphite (ca.0.335 nm).In addition, the FTIR spectrum shows typical peaks of O-H bonding, C=O bonding, C=C bonding, C-OH bonding and C-O bonding.The characterization results before and after testing are consistent, demonstrating no chemical or structural changes.Figures S17-S19 show the EDS, Raman, and X-ray Photoelectron Spectroscopy (XPS) spectrum of GO in the initial state of GZMEG and after 100 ME test cycles.After the performance cycle test, no obvious change in the atomic ratio of C and O of the GO film is found (Fig. S17).Raman spectrum are a widely used method for characterizing graphene-based materials and the Raman spectrum of GO shows an apparent disorder D band at 1340 cm -1 and graphitic G band at 1600 cm -1 [S4].The ID/IG ratios of the GO in the initial state and after the cycle test are 0.937 and 0.939, respectively, indicating no apparent structural change happens during the cycling test.Furthermore, the high-resolution C 1s spectrum (Fig. S19b) of GO revealed the presence of C-C bonding (~284.5 eV), C-O bonding (~286.8eV), and C=O (~288.5 eV) [S1], and the spectrum after the test is basically unchanged, indicating that the oxygen-containing groups are still retained.
The SEM images (Fig. S20) of the ZnO layer display no significant change in the microscopic morphology before and after 100 cycles of ME testing, and the XRD pattern and FTIR spectrum are shown in the Figs.S21 and S22.There is no change in the characteristic peaks of ZnO with hexagonal wurtzite structure, implying that the ZnO layer was stable on GZMEG.The sharp peaks at 424 and 560 cm -1 in the FTIR spectrum are the characteristic peaks of Zn-O bonding.The absorption peaks at 3440 and 1630 cm -1 were attributed to the stretching and bending vibration absorption peaks of ZnO surface hydroxyl groups or bridged hydroxyl groups.In the atmosphere at room temperature, water is adsorbed on the surface of metal oxides, and in most cases the water eventually dissociates to form adsorbed Nano-Micro Letters S4/S24 hydroxyl groups.The above results show that the ME effect did not change the chemical composition of GO and ZnO, which means that the GZMEG has long-term stable performance.
To verify that external factors such as electrodes would not affect the ultrafast response characteristics of GZMEG, we prepared GZMEGs with different electrodes and stimulated them by moisture carried by different atmospheres.The GZMEGs with gold, FTO conductive glass, and CH8 conductive carbon paste as electrodes, respectively, show similar rapid response capabilities (Fig. S23).Among them, the peak value of the electrical signal of GZMEG with CH8 conductive carbon paste as the electrodes is slightly smaller than other GZMEGs, because the carbon paste is not as smooth as other electrodes, and the contact with GZMEG is relatively poor, which will affect the output signal.On the other hand, changing the N2 source to argon or air to adjust the RH, GZMEG also has a fast response performance (Fig. S24).The results indicate that external factors could not affect the characteristics of GZMEG.
The voltage and current responses of GZMEG under different RH conditions are shown in Figs.
S25 and S26 respectively.All tests were performed by alternately stimulating GZMEG with moisture at different RH and dry N2.Under 10%-40% RH, the voltage output is close to 0, because the resistance of ZnO is large under low RH, which limits the voltage output (Fig. S3f).As the RH further increases, the output voltage of GZMEG gradually increases and the response to the stimulation of alternating moisture becomes more rapid, since the Grotthuss mechanism more obviously promotes the directional diffusion of the ionized H + in GO, and the resistance of ZnO is smaller under high RH, which has little limiting effect on voltage.It is worth noting that the current has the same performance as the voltage under different RH, but the current is 0 below 50%RH owing to the output of the current signal is more easily restricted by the resistance of the external circuit (Fig. S3f).
The output performance of GZMEGs with different coverage areas was tested with other conditions being the same, where the ZnO layer covered 25%, 40%, 50%, 60% and 75% of the GO film area, as shown in (Fig. S27).The GZMEGs exhibited excellent rapid response to alternating stimulus of moisture (90% RH) and dry N2, but the magnitude of voltage and current showed a trend of first increasing and then decreasing with the increase of coverage area.The smaller output signals of GZMEG with 25% and 40% coverage area were attributed to less ZnO in the upper layer, resulting in the Grotthuss proton hopping phenomenon not being strong enough to encourage the diffusion of abundant H + .The difference in the signal peaks for GZMEGs with 60% and 75% coverage areas was attributed to the greater intrinsic resistance of more ZnO, which bounds the signal output.
Different sizes of GZMEGs were prepared and their voltage and current responses are shown in Fig. S28.The sizes of GO and ZnO layers in the GZMEGs prepared in this work are 1×1 cm 2 , and the dimensions of the two functional layers in the GZMEG from the control experiments are 0.5, 2, and 3 cm 2 , respectively.There is no significant difference in the voltage responsivity and magnitude of the different sizes of the GZMEGs, as the potential difference between the upper and lower surfaces of the GZMEGs is unchanged.However, the current magnitude of GZMEGs is proportional to the device size.As the size increases, the number of directionally migrating H + increases with it, and a larger current is generated.The current density is the same for all GZMEGs.

Note S5 Working Mechanism of GZMEG
A ZnO flake with a diameter of 13 mm was prepared by a powder tablet machine, and then a drop of GO aqueous dispersion (0.2 mg/mL) was dropped on it and then dried at 35 ℃ for Kelvin probe force microscopy (KPFM) test (Fig. S33a).The SEM images show that the ZnO flake has a very flat surface, and a thin layer of GO adheres to the ZnO (Fig. S33b, c).The relative surface potential Nano-Micro Letters S5/S24 was measured at the junction of the GO-ZnO heterostructure (Fig. S33d), which is the red frame in Fig. S33a.The test results are shown in Fig. 3h.
With the overall structure unchanged, GO-Al2O3 ME generator (GAMEG), GO-Fe3O4 ME generator (GFMEG), and GO-TiO2 ME generator (GTMEG) were prepared by replacing ZnO with Al2O3, Fe3O4, and TiO2 to generalize "built-in interfacial potential coordination of heterostructures" theory to other moisture-electric-moisture-sensitive heterostructures.The electrical output properties of Al2O3, Fe3O4, and TiO2 under 90% RH demonstrate that these oxides do not have ME characteristics (Fig. S34).Their resistance varied rapidly with RH to ensure fast MEG response.The difference in maximum current and voltage is attributed to the difference in the intrinsic resistance of the moisture-sensitive oxides (Fig. S35).The resistances of Al2O3, Fe3O4, and TiO2 are 25, 6, and 5.4 MΩ at 90% RH, and 10 10 , 10 9 , and 6×10 8 Ω under 10% RH, respectively.The electrical signals of the moisture-electric-moisture-sensitive MEG is shown in Figs.S3e and S3f to decreases with the increase of the external series resistance, and the moisture-sensitive oxide in the moisture-electric-moisture-sensitive MEG is equivalent to a series external resistance.At the same high RH (90%), the resistance of Al2O3 is ten times that of ZnO, so the output voltage and current of GAMEG are lower.

Note S6 Application of GZMEG as an HRM
Using the GZMEG-based HRM, a series of devices for judging human respiration status and monitoring respiratory diseases in real-time were developed, including a respiration indicator light, apnea alarm, and obstructive sleep apnea-hypopnea syndrome (OSAHS) diagnostic system.A very portable and simple HRM consists of a GZMEG, a mask, and gold electrodes, as shown in Fig. S38.This simple respiration monitor can be used for various functions.For example, by connecting an amplifier and an LED, the GZMEG can be used as a respiratory indicator (Fig. S39a).The LED could respond in sync with the respiration to indicate the respiration status.The apnea alarm is composed of GZMEG, data acquisition, voltage comparison, LCD screen, LED, and buzzer (Fig. S39b).When the apnea time exceeds 30 seconds, the alarm indicator lights and the buzzer sounds.In addition, we also developed an OSAHS diagnostic system, which successfully monitored the respiratory status within one hour in real time (Fig. S40).

Introduction to Movies
Movie S1: Water contact angles of ZnO flake and GO film.ZnO is super hydrophilic with a water contact angle close to 0°.GO is hydrophilic with a water contact angle of 51°.Movie S4: Apnea monitoring by GZMEG-controlled apnea alarm.This movie demonstrates the successful monitoring of apnea for more than 30 s by the alarm.
Movie S5: Judgment of respiratory status by OSAHS diagnostic system based on GZMEG.This movie displays that the system can distinguish between normal breathing, hypopnea, and apnea, and calculate the AHI to diagnose OSAHS.

Fig. S1
Fig. S1 a) Photograph of GO film.b) Dimensional diagram of the porous gold electrode.c) Photograph of a porous gold electrode Fig. S1 a) Photograph of GO film.b) Dimensional diagram of the porous gold electrode.c) Photograph of a porous gold electrode

Fig. S9
Fig. S9 Electrical output performance of ZnO.a) Schematic diagram of testing the electrical output performance of ZnO.Output voltage (b) and current (c) of ZnO under RH of 90%

Fig. S11
Fig. S11 Water contact angles of ZnO flake (a) and GO film (b)

Fig. S17
Fig. S17 EDS spectrum of GO film in GZMEG at the initial state and after 100 cycles of ME testing

Fig. S23
Fig. S23 Voltage and current responses of GZMEG with different electrodes.(a, b) Gold electrode.(c, d) FTO conductive glass electrode.(e, f) Conductive carbon paste electrode.Moisture at 90% RH was used for all humidification processes, and dry N2 was used for dehumidification processes

Fig. S27
Fig. S27 Voltage and current responses of GZMEGs with different coverage areas, moisture at 90% RH was used for all humidification processes, and dry N2 was used for all dehumidification processes

Fig. S28
Fig. S28 Voltage and current responses of GZMEGs with different sizes, moisture at 90% RH was used for all humidification processes, and dry N2 was used for all dehumidification processes

Fig. S30
Fig. S30 Schematic diagram of the current and voltage output mechanism of GOMEG

Fig. S37
Fig. S37The preparation process of the integrated GZMEG units by step-by-step screen printing

Movie S2 :
Comparison of the responsiveness of GOMEG and GZMEG to moisture.This movie demonstrates the response to the moisture of LEDs controlled by GOMEG and GZMEG, respectively.Movie S3: Real-time responses of a respiration indicator light controlled by GZMEG to human respiration.This movie shows the real-time response of the respiration indicator light controlled by GZMEG to breathing.The indicator light is off when inhaling, and the indicator light is on when exhaling.The breathing status is monitored in real-time by this method.

Table S1
Comparison of voltage responses of different MEG systems

Table S2
Comparison of different MEG systems for respiratory monitoring